Multiple sclerosis (MS) is a chronic disease of the central nervous system (CNS), with an uncertain cause. Colleen Hayes and Donald Achaeson have suggested that insufficient sunlight exposure and chronic viral infections might be unrelated environmental risk factors for MS. These risk factors may act synergistically to enable the pathogenic autoimmune response.

The prevalence of MS is highest where environmental supplies of vitamin D are lowest. Sunshine enables the production of vitamin D3 (VD3) in the skin. Epidemiological studies have shown that higher vitamin D blood levels are associated with lower risk, less relapses and a slower progression of multiple sclerosis. Higher vitamin D levels can be achieved in part by increased oral intake of VD3.